Many liquid crystal displays are illuminated by a backlight module including a linear strip of light emitting diodes, or LEDs. Typically, LEDs are positioned at the edges of the display and light from the LEDs is guided by a light guide. The light guide usually incorporates small scattering structures to scatter the light in a forward direction of the display. Sometimes, a reflective back surface is used to enhance light recycling by reflecting light in the forward direction. The scattering structures are typically designed to maximize scattering efficiency in the forward direction and also to ensure that the backlight has a uniform brightness across the display.
Many commercial displays use brightness-enhancing films after the backlight in order to collimate the light from the backlight with the goal of increasing on-axis brightness for the display. This not only improves on-axis brightness, it also increases the ON-OFF contrast of the display, improves performance of polarizers which may be used, and allows for private viewing of mobile displays. However, the use of separate optical films for collimating the light tends to decrease the overall efficiency of the display because some light is absorbed before leaving the display. Scattering structures in the light guide can also decrease the efficiency of the display by causing some light to be absorbed or otherwise dissipated.